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Influenza A Virus in Swine Surveillance

Fiscal Year 2018 Quarterly Report

Surveillance Summary for Fourth Quarter Fiscal Year 2018: July 1 to 

September 30, 2018

Report Summary1

 This report covers the fourth quarter (Q4) of fiscal year 2018 from July 1 to September 30, 2018.

 Where relevant, the report also includes previous years’ data for historical perspective. 

 The report provides data from both national and regional levels.

 In fiscal year 2018, there were 28,525 samples submitted for influenza A virus (IAV) surveillance 

in swine from 9,765 accessions. 

 H1N1 was the predominant subtype reported in USDA data in 2018.

 Over the past 7 quarters, H1N2 was the main subtype in Region 1 while Region 2 was H1N1, 

Regions 3 and 4 were H3N2 and Region 5 most frequently isolated H1N1 and mixed subtypes, 

respectively. For regions recorded as “unknown,” H1N2 was the most frequent subtypes.

 Limited accessions from a region can skew data and lead to misinterpretation. Therefore, less 

inference can be applied to results from Regions 3, 4, and 5.

 All IAV-S submissions are voluntary and based on clinical case submissions to veterinary 

diagnostic labs. These data are not a statistically representative sampling of the U.S. swine 

population.

 Due to the voluntary nature of this surveillance, the information in this report cannot be used to 

determine regional and/or national incidence, prevalence, or other epidemiological measures, 

but it may help identify IAV-S trends. 

Introduction

This report, based on data received as of February 24, 2020, provides a brief update on the status of 

national surveillance for IAV in swine for producers, swine practitioners, diagnosticians and the public. 

Summaries in this report may differ from those provided in past reports due to the regular addition of 

data from participating laboratories. Reporting months are based on the month the sample was 

collected. The IAV-S surveillance program is voluntary and, as a result, the accessions and samples 

submitted represent a subset of the swine population. Submitted samples should only be collected from 

 

1

In November 2016, VS modernized the process that prepares and stages laboratory results data for reporting. Consequently, 

VS recognizes there is a small difference in previously reported summary numbers for IAV-S surveillance. The results in this 

report reflect updated and corrected numbers achieved with the modernized data process.

Animal and 

Plant Health 

Inspection 

Service

Veterinary 

Services

February 2020
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animals displaying influenza-like illness. Due to its voluntary nature, this surveillance system does not 

entirely represent the total U.S. domestic swine population. Therefore, the data cannot be used to 

determine IAV-S prevalence or other epidemiologic measures in the swine population. However, the 

data may help identify trends of influenza in swine. 

When the submitter does not report relevant information, data are recorded as “unknown.” Summaries 

in this report may differ from those provided in past reports due to the ongoing addition of data from 

participating labs. Reporting months are based on the month when the sample was collected. 

A laboratory accession is generally a set of samples collected at a single premises on a single day and 

received at the laboratory. A maximum of 10 samples of any kind is allowed per accession for 

reimbursement under the USDA IAV-S system. However, no more than five of the 10 samples may be 

oral fluid for any given accession. This does not prevent additional samples from being tested at the 

owner’s expense. While a nasal swab or lung tissue sample represents a single animal within the herd, a 

single oral fluid sample may represent one to two pens of animals in a herd. A positive sample status is 

based on the screening real-time reverse transcriptase polymerase chain reaction (rRT-PCR). The 

subtype result is based on the rRT-PCR-based subtyping assays. Virus isolation (VI) and sequencing in the 

NAHLN labs are only attempted on rRT-PCR positives meeting criteria with sequences deposited into 

GenBank, the public sequence database. On a monthly basis, USDA also performs whole genomic 

sequencing (WGS) on a selected subset of virus isolates received through surveillance and deposits 

those sequences into Genbank. On a quarterly basis, a phylogenetic analysis is performed; phylogenetic 

analyses are based on all successful USDA surveillance sequencing results, with sequences deposited 

into GenBank®, the public sequence database.

Program Updates

There have been recent changes to the USDA’s webpages that provide information on IAV-S and IAV-S 

surveillance. Please visit the new webpage and navigate around to find the information you may be 

seeking as well as the quarterly IAV-S reports. The new web address is:

https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/swine-diseaseinformation/influenza-a-virus

The focus of IAV-S surveillance remains on acquiring viruses. The National Animal Health Laboratory 

Network (NAHLN) has several submission options to ensure unusual viruses identified by methods other 

than those approved for NAHLN testing can be submitted into the program. An updated version of the 

IAV-S NAHLN testing guidelines and instruction sheet can be found at:

https://www.aphis.usda.gov/animal_health/animal_dis_spec/swine/downloads/appendix_c_testing_gui

delines.pdf and 

https://www.aphis.usda.gov/animal_health/animal_dis_spec/swine/downloads/iav-s-algorithminstructions.pdf

IAV-S Surveillance Objectives

USDA’s National Surveillance Plan for Swine Influenza Virus in Pigs (July 2010) describes the current 

surveillance system for IAV in swine in detail. The surveillance objectives are to: 

1. Monitor genetic evolution of endemic IAV in swine to better understand endemic and emerging 

influenza virus ecology; 
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2. Make influenza isolates from swine available for research and establish a data management system to 

facilitate genetic analysis of these isolates and related information; and 

3. Select proper isolates for the development of relevant diagnostic reagents, updated diagnostic assays, 

and vaccine seed stock products. 

Objective 1. Monitoring Genetic Evolution of Endemic IAV in Swine to Better 

Understand Endemic and Emerging Influenza Virus Ecology

Objective 1 is met through the submission of diagnostic laboratory samples to the surveillance system, 

collection of the viruses that are isolated from the samples, and analysis of the hemagglutinin (HA) and 

neuraminidase (NA) sequences that are generated at the NAHLN laboratories. Each month, selected

viruses undergo whole genomic sequencing by the National Veterinary Services Laboratories (NVSL). 

Phylogenic analysis of the genetic sequences submitted through the surveillance program is provided 

through an interagency agreement with the USDA’s Agricultural Research Service (ARS) National Animal 

Disease Center (NADC). 

National Surveillance Data Summary

From fiscal year 2010 through fiscal year 2016, the total number of accessions and samples submitted 

increased. Changes initiated in fiscal year 2016 resulted in decreased laboratory accessions and samples,

but a higher percentage of accessions resulting in a virus isolate that can be sequenced and analyzed. In

fiscal year 2018, 9,765 samples have been tested from 28,525 accessions (Figure 1). Figure 2 shows the 

overall increasing trends in total accessions, rRT-PCR and VI positive accessions, and subtyped 

accessions.

Figure 1. Number of IAV-S laboratory accessions and samples tested in swine, fiscal year 2011 through fiscal 

year 2018 Q4.
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Figure 2. Accessions submitted, subtyped accessions, rRT-PCR positive accessions, and virus isolation positive 

accessions over time with trend lines for influenza A virus in swine, fiscal year 2011 to fiscal year 2018 Q4.

Figure 3 shows the number of subtype detections in fiscal year 2018 Q4. The total number of samples

subtyped was 225, including H1N1 (n=73), H1N2 (n=81), H3N2 (N=71) and mixed (N=4). H3N1 was not 

isolated.

Figure 3. Number of influenza A subtype detections in swine fiscal year 2018 Q4.

Figure 4 breaks down accessions by rRT-PCR subtype from fiscal year 2011 Q1 to fiscal year 2018 Q4. 

H1N1 was the predominant subtype detected in 2012, 2013, 2014 and 2018. H1N2 was detected most 

often in 2011, 2015, 2016, and 2017.
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Figure 4. Number of subtypes, fiscal year 2011 through fiscal year 2018 Q4.

Figure 5 displays the number of times VI was conducted in blue, the number of times VI was conducted 

and was positive in purple, and the number of viral isolates submitted to GenBank in green. Since the 

implementation of the June 2016 modifications to the program, almost all VIs attempted now yield a 

virus and the sequences are submitted to Genbank for analysis.

Figure 5. Number of influenza A virus isolations attempted, number of virus isolations that were positive, and 

the number of viruses submitted to GenBank from fiscal year 2011 to fiscal year 2018 Q4.

Laboratory accessions were evaluated by age-class for the first quarter. The most common subtype 

isolated among the nursery age class was H3N2 and grow- finish age classes was H1N1. Among suckling 

age class, H1N2 was the predominant isolated subtype. The sow/boar class had limited testing, with one 

isolate of H1N2. Among isolates for which the age class was unknown or not recorded, H1N2 and H3N2 

were the predominant subtypes (Table 1). Excluding specimen types with less than 10 percent of total 

submissions, samples collected from nasal specimens (swabs or wipes) were the most successful at 

providing a virus isolate which enables sequence submission to GenBank (Table 2).
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Table 1. Number of positive accessions tested for influenza A virus in swine by age class and subtype fiscal year

2018, Q4.

Age class Number of accessions with 

subtype reported

Number 

of H1N1

Number 

of H1N2

Number 

of H3N1

Number 

of H3N2

Number 

of Mixed

Suckling 24 5 10 0 8 1

Nursery 79 23 26 0 29 1

Sow/Boar 2 0 1 0 1 0

Grow/finish 67 31 22 0 13 1

Unknown 55 14 20 0 20 1

Table 2. Number of positive accessions* tested for influenza A virus in swine by specimen type** and subtype fiscal 

year 2018 Q4.

Specimen 

Type

Number of 

accessions 

with 

subtype 

reported

Percent of 

subtyped 

accessions with 

positive virus 

isolation

Number 

of H1N1

Number 

of H1N2

Number 

of H3N1

Number 

of H3N2

Number 

of Mixed

Number of 

samples 

sequenced 

and sent to 

GenBank

Lung 189 92% 71 66 0 50 2 175

Nasal 30 93% 1 9 0 18 2 27

Oral Fluid 8 88% 1 4 0 3 0 7

Other 2 100% 0 2 0 0 0 2

*Accessions may include samples with multiple specimen types. In these cases, individual accessions are counted 

in more than one specimen type category. 

** Other includes specimen types recorded as swab, mixed tissue, or unknown.

Regional surveillance data

In this section, we present data in five different regions (Figure 6). These regions are based on current 

USDA administrative districts only and do not represent specific industry distribution. Submissions are 

voluntary, as is any identifying information accompanying the submission (except the State of animal 

origin), and therefore no sampling strategies can be applied to the regions.
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Figure 6. A map of the regions for national IAV-S surveillance.

Summary of Regional Data from ARS

Table 3. Summary of predominant subtypes in each region in a 2 year window (fiscal year 2017

Q1 through fiscal year 2018 Q4).

Region Total number HA/NA subtype

1 253 Gamma H1/Classical N1

Delta2 H1/1998-N2

Human-like_2010.1 H1/2002-N2

2 1,197 Gamma H1/Classical N1

Human-like_2010.1 H3/2002-N2

Delta 1a H1/2002-N2

3 152 Delta 1a H1/2002-N2

Gamma H1/Classical N1

Human-like_2010.1 H3/2002-N2

4 163 IV-A H3/2002-N2

Gamma H1/Classical N1

Delta1b H1/2002-N2

5 14 Low participation

Human-like_2010.1 H3/2002 N2

Pandemic H1/Pandemic N1

Delta 1a H1/1998-N2

Most Predominant HA/NA phylo-types overall: H1N1 (Gamma H1/Classical N1), H3N2 (Humanlike_2010.1 H3/2002-N2) and H1N2 (Delta1a H1/2002-N2) 

Figure 7 shows the distribution of rRT-PCR subtyped accessions among the five regions for Q1 fiscal year

2017 through Q4 fiscal year 2018. Over the past 7 quarters, H1N2 was the main subtype in Region 1 

while Region 2 was H1N1, Regions 3 and 4 were H3N2 and Region 5 most frequently isolated H1N1 and 

mixed subtypes. For regions recorded as “unknown,” H1N2 was the most frequent subtypes.

Region 5 Region 3 Region 1
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Figure 7. Percentage of influenza A subtyped accessions for swine by region for fiscal year 2017 Q1 to fiscal year 

2018 Q4.

Regional phylogenetic analysis

Phylogenetic analysis of sequences from the IAV-S surveillance system

Phylogenetic analysis of gene sequences of IAV in swine is conducted of this rapidly changing virus. 

Through collaboration with ARS, a dataset2,3 of 342 isolates with published to further examine the 

genetic changes that occur in HA, NA, and Matrix (M) gen sequences in GenBank was characterized by 

phylogenetic analysis for the Q4 fiscal year 2018 report. This analysis provides information on the 

genetic diversity and evolution patterns of influenza in swine and allows for inferences about population 

and/or vaccine immunity.

The following series of bar charts parse the data into an approximately 2-year window by quarters and 

subtypes for each region, followed by charts further describing the phylogenetic clades of H1 and H3 

subtypes. Regional charts depicting the various combinations of HA and NA are in Appendix 1. 

 

2 Participating NAHLN labs included M gene sequencing in their testing until July 2016 because the 2009 H1N1 M gene was the 

predominant circulating gene.

3

The ARS dataset is comprised of IAV-S surveillance isolate sequences that were posted in Genbank. This represents only a 

subset of the complete IAV-S surveillance dataset that includes PCR diagnostic test-based results as well as sequencing results. 

Therefore, ARS dataset results, such as subtype percentages, differ from the complete IAV-S dataset results provided in other 

sections of this report.
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Figure 8. Temporal distribution of Influenza A virus subtype by region for Q1 fiscal year 2017 to Q4 fiscal year 

2018

Figure 8 demonstrates the four subtypes H1N1, H1N2, H3N2 and mixed subtypes across the five regions. 

Regions 1 and 2 reported the most submissions, with a mixture of mostly H1N1, H1N2, and H3N2. 

Limited accessions from a region can skew data and lead to misinterpretation and therefore, less 

inference can be applied to results from Regions 3, 4, and 5.

National phylogenetic HA gene information 

HA genes from H1 subtype viruses are classified as alpha, beta, gamma, delta-1, delta-2, or pandemic 

H1N1 2009 (H1N1pdm09) phylogenetic clades based on a previously published nomenclature system. 

Similarly, H3 subtype viruses are classified as Cluster IV, Cluster IV-A, Cluster IV-B, Cluster IV-C, Cluster 

IV-D, Cluster IV-E, Cluster IV-F, or human-like. In Q4, fiscal year 2018 there were 118 H1s detected. Of

the H1 subtypes (Figure 9), gamma, delta 1a and delta 2 were the predominant H1 subtypes detected

with continued detection of alpha with 2 aa deletions. Gamma2-beta-like was detected in Iowa, 

Minnesota, Oklahoma, Nebraska, Illinois and South Dakota.
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Figure 9. Temporal distribution of H1 phylogentic clades by region for Q1 fiscal year 2017 to Q4 fiscal year 2018

 

Figure 10. Temporal distribution of H3 phylogentic clades by region for Q1 fiscal year 2017 to Q4 fiscal year 2018

In Q4 fiscal year 2018, there were 53 detections of H3s (Figure 10). Human-like 2010.2 H3 was detected

in Arkansas, Oklahoma and Indiana. Cluster I H3 was detected in Minnesota, Tennessee and Iowa paired 

with N2.TX98. The predominant H3 was Human-like_2010.1 H3.
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National phylogenetic NA gene information 

Whole genome patterns with HA/NA pairs were reported and the dominant patterns are in descending 

order below with most frequent pattern by HA/NA pair: 

 TTTTPT – H3.2010.1/N2-02, H1.delta1a/N2-02, H1.gamma/cN1, 

 TTTPPT – H1.delta2/N2-98, H1.gamma/cN1, H1.delta1a/N2-02

 TTPPPT – H1.gamma/cN1, H1.alpha/N2-02, H1.delta1a/N2-02

Vaccine constellations include VVVVVV, VTVVPT, TVVVVT, TVVVPT, TVVPTP, TTVTPT and TTVPPT

Both the N1 and N2 subtypes are found in circulating swine viruses. Classical N1 continued to be the 

dominant cluster at 88 percent; and the 2002-lineage N2 represents 80 percent of N2 collections. 

Objective 2. Make Influenza Isolates from Swine Available for Research and 

Establish a Data Management System to Facilitate Genetic Analysis of these 

Isolates and Related Information

A primary goal of IAV swine surveillance is to share selected virus isolates obtained through the 

surveillance system with public health, animal health, and academic researchers to facilitate genetic 

analysis and research on viruses of interest. The NVSL Diagnostic Virology Laboratory maintains a 

repository of the viruses submitted into the surveillance system and provides these viruses upon 

request. 

In the fourth quarter of fiscal year 2018, the NVSL Diagnostic Virology Laboratory provided 48 isolates to 

one government and four international institutions. NVSL received 280 isolates into the repository 

(Table 4). Table 5 reports the total number of isolates available in the repository by subtype for sharing. 

Table 4. Virus isolates received in repository. 

Virus isolates in the repository

2018 994

2017 844

2016 1,046

2015 883

2014 765

TOTAL TO DATE 4,532

 

 

Objective 3. Select Proper Isolates for Development of Relevant Diagnostic 

Reagents, Updating Diagnostic Assays, and Vaccine Seed Stock Products

USDA makes IAV-S isolates available in the public domain for further research. ARS-NADC conducts 

research on isolates obtained from the repository and sequences generated from the surveillance 

system. Genetic sequencing from the USDA program that is reported to GenBank is available for private 

corporations, government entities, academia, and other scientific community partners for research and 

vaccine strain selection and efficacy testing. NVSL and ARS staff are consulted as subject matter experts 

when necessary. 
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Conclusion

The IAV voluntary surveillance system in swine continues to provide insight into the genetic makeup of 

circulating influenza A virus in limited populations of commercial pigs. Genetic information and virus 

isolates are made publicly available for further research and possible vaccine strain selection and 

efficacy testing. Influenza A virus in swine remains a dynamic virus with high levels of genetic variability 

in the hemagglutinin and neuraminidase genes. 

Table 5. Total number of subtyped 

isolates available through repository.

Subtyped isolates available through 

repository

H3N2 1,674

H3N1 16

H1N1 2,193

H1N2 1,971

Mixed 300

TOTAL 6,154
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Appendix 1. Regional Charts of HA and NA Combinations by Percentage

The following charts present the percentages of combinations of HA and NA on the national and regional scales based on ARS-NADC phylogenetic 

analyses. The results are reported from October 2016 to September 2018. These “heat maps” represent the percentage of combinations by using 

a color gradient where a deeper gradient color represents a greater percentage occurrence for a particular HA-NA combination. HA clusters are 

listed on the left vertical axis of the chart and NA clusters are listed on the bottom horizontal axis. Line up the HA cluster with the corresponding 

NA cluster to determine the occurrence of that particular combination. 
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Region 1. Total HA & NA combinations –253
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Region 2. Total HA & NA combinations –1,197
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Region 3. Total HA & NA combinations - 152
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Region 4. Total HA & NA combinations – 163
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Region 5. Total HA & NA combinations – 14